1. Field of the Invention
The present invention relates to an image reading apparatus and a correction method thereof, and particularly to, for example, an image reading apparatus that optically reads an image original using a plurality of CISs (contact line sensors) and a correction method thereof.
2. Description of the Related Art
Conventionally, a large format image reading apparatus (to be referred to as large format scanner hereinafter) for reading a large original is used to read the image of the drawing of a large building or a large machine, a large map, a large poster, or the like and store the data in a storage device or copy and print based on the data on a printing medium such as a paper sheet.
The large format scanner generally uses the image reading element of an A4 scanner that is widely available in the market, for example, a line-type reading sensor having a reading width corresponding to A4. This small line-type reading sensor has unreadable areas on both sides. To acquire a seamless image, the small line-type reading sensors are arranged zigzag, and images are connected after reading, thereby generating one large format image.
FIG. 11 is a block diagram showing the schematic arrangement of a conventional image reading apparatus.
A large format scanner uses a plurality of line-type reading sensors depending on the model difference in accordance with the size of an original to be read. An example in which three small CISs (contact image sensors) are used will be described here for the sake of simplicity. In FIG. 11, the CISs are expressed as CIS_1, CIS_2, and CIS_3.
A case where CISs are used as line-type reading sensors will be explained below.
Three CISs 2a, 2b, and 2c that are line-type reading sensors are arranged behind a conveyance roller 1 for conveying an original. An image is read at the positions of the CISs arranged zigzag. Each CIS output passes through a pixel preprocessing unit 3 formed from an A/D conversion unit, a CIS rearrangement unit, and a shading unit that should adjust the monochrome reference value, and is then input to an image processing unit 4. On the other hand, a roller angle detection unit 10 configured to detect a rotation position is provided for the conveyance roller 1. The conveyance roller 1 is connected to a motor 11 via a belt pulley so as to be rotatable by an original conveyance shaft drive unit 12 and form an original convenience unit. The roller angle detection unit 10 is connected to various drive control units 8 in a circuit board together with the original conveyance shaft drive unit 12 and various sensors 13. Image reading is performed while controlling the CISs 2a, 2b, and 2c and storing data in a memory unit 5.
In recent years, image reading needs to be done more accurately. In the image reading apparatus having the above arrangement, since a long conveyance roller is used, the bend of the roller shaft is not negligible, and the bend adversely affects image reading.
FIGS. 12A to 12D are views showing images expressed by image data obtained by reading a calibration sheet with slant lines printed on it and stored in the internal memory unit of the image reading apparatus. In the images shown in FIGS. 12A to 12D, the horizontal direction is the direction (main scanning direction) in which the reading elements of the CISs are arrayed, and the vertical direction is the original conveyance direction (sub-scanning direction). Note that the original may be actually conveyed by an ADF or the like or relatively conveyed by moving the CIS unit itself.
FIG. 12A shows an image represented by read image data 14 of CIS_1. Similarly, FIG. 12B shows an image represented by the read image data 14 of CIS_2, and FIG. 12C shows an image represented by the read image data 14 of CIS_3. Referring to FIGS. 12A to 12D, the alternate long and short dashed lines indicate slant lines that should originally be. If the conveyance roller 1 has an axis bend, as shown in FIG. 11, deviations occur in the sub-scanning direction with respect to the slant lines that should be, as shown in FIGS. 12A to 12C. As a result, images represented by the thick solid lines are obtained.
A vertically long rectangular portion 14a1 shown in FIG. 12A represents a CIS connection position of one of the CISs arranged zigzag. Similarly, FIG. 12B shows CIS connection positions 14b1 and 14b2 on both sides of a CIS, and FIG. 12C shows a CIS connection position 14c1 on the left side of a CIS. When the three images shown in FIGS. 12A to 12C are composed in this state, a composed image shown in FIG. 12D is obtained.
Discontinuity of slant lines occurs at the CIS connection positions, as indicated by 14d1 and 14d2 in FIG. 12D. In addition, the overall linearity becomes poor, as indicated by an arrow 14d3 in FIG. 12D. As described above, the challenge is to improve the accuracy after image composition. Concerning this challenge, an attempt has conventionally been made to solve the shift after image composition by contriving the reading timing of the image reading apparatus. For example, Japanese Patent Laid-Open No. 2009-246671 proposes determining the original conveyance position and starting the image reading operation based on the determination result to improve the reading accuracy at the time of acceleration/deceleration of CISs. Japanese Patent Laid-Open No. 2011-044783 discloses an arrangement limited to monochrome reading, in which the original conveyance position at the time of acceleration/deceleration of CISs is determined, and a reading activation pulse is output, as in Japanese Patent Laid-Open No. 2009-246671.
Japanese Patent Laid-Open No. 2009-246671 describes that original position detection is done not directly but using a signal from an encoder provided on the conveyance roller or the like, and the actual original conveyance position and the encoder signal have a difference. However, there is not disclosed an arrangement that changes the reading timing in the main scanning direction of reading in consideration of decentering that changes depending on the axial portion of the long conveyance roller.
Japanese Patent Laid-Open No. 2011-044783 is directed to reading of a monochrome original, and proposes that, if actual original conveyance has not reached the target, the data is discarded, and next data is employed. However, there is not disclosed an arrangement that focuses on the difference in the original conveyance speed in the main scanning direction.
As described above, the large format scanner uses a long conveyance roller. For this reason, decentering occurs in the direction of the rotation axis of the conveyance roller. The original conveyance speed is not even in the direction of the rotation axis. In addition, since the strict reading position changes in the main scanning direction of CISs, a drawn straight line on the original cannot be read as a straight line.
In the conventional large format scanner, a plurality of CISs that have a short reading width and are originally used in a scanner for reading an A4 original or the like are arranged zigzag to partially overlap the reading position in the main scanning direction. After image reading, images read by the CISs are composed. At this time, averaging processing is performed using a calibration sheet, and the images of the CISs arranged zigzag are composed. However, since the conveyance roller bends, the images at the CIS connection portions may shift at a certain instant.